Kinetics and mechanism of the oxidation of iron(II) ion by chlorine dioxide in aqueous solution

The mechanism by which an excess of iron(II) ion reacts with aqueous chlorine dioxide to produce iron(III) ion and chloride ion has been determined. The reaction proceeds via the formation of chlorite ion, which in turn reacts with additional iron(II) to produce the observed products. The first step of the process, the reduction of chlorine dioxide to chlorite ion, is fast compared to the subsequent reduction of chlorite by iron(II). The overall stoichiometry is The rate is independent of pH over the range from 3.5 to 7.5, but the reaction is assisted by the presence of acetate ion. Thus the rate law is given by At an ionic strength of 2.0 M and at 25°C, k_u = (3.9 ± 0.1) × 10³ L mol^?1 s^?1 and k_c = (6 ± 1) × 10⁴ L mol^?1 s^?1. The formation constant for the acetatoiron(II) complex, K_f, at an ionic strength of 2.0 M and 25°C was found to be (4.8 ± 0.8) × 10^?2 L mol^?1. The activation parameters for the reaction were determined and compared to those for iron(II) ion reacting directly with chlorite ion. At 0.1 M ionic strength, the activation parameters for the two reactions were found to be identical within experimental error. The values of ΔH? and ΔS? are 64 ± 3 kJ mol^?1 and + 40 ± 10 J K^?1 mol^?1 respectively. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 554–565, 2004     

Characterization of Anilinepentacyanoferrate (II) and (III)

The anilinepentacyanoferrate (II) complex has been characterized in aqueous solution. The complex exhibits a predominant ligand field transition at λ_max = 415 nm with ?_max = 494 M^?1 cm^?1. The corresponding Fe(III) complex displays a strong absorption at λ_max = 700nm(?_max = 1.61×10⁴ M^?1 sec^?1) which can be assigned as a ligand to metal charge transfer transition. The rate constants of formation and dissociation for the Fc(II) complex are (3.14±0.18)×10² M^?1W^?1 and 0.985±0.005 sec^?1, respectively, at μ = 0.10 M LiClO₄, pH = 8 and T = 25°C. The cyclic voltammetry of the complex shows that a reversible redox process is observed with E_1/2 value of 0.51±0.01 V vs. NHE at μ = 0.10 M LiClO₄, pH = 8 and T = 25°C. The kinetic study of the oxidation of the Fe(II) complex by ferricyanide ion yielded the rate constant of the reaction k_et = (1.43±0.04)x10 M sec^?1 at μ = 0.10 M LiClO₄, pH = 8 and T = 25°C.     

The ultraviolet absorption spectra of the acetyl radical and the kinetics of the CH3 + CO reaction at room temperature

A continuum-absorption spectrum between 200 and 240 nm is assigned to the acetyl radical. Kinetic measurements using molecular modulation spectroscopy show for the reaction CH₃ + CO (+M) → CH₃CO + M the rate constants are (1.8 ± 0.2) × 10^?18 cm³ molecule^?1 s^?1 at 100 Torr and (6 ± 1) × 10^?18 at 750 Torr. The rate constant for acetyl combination 2CH₃CO → (CH₃CO)₂ is (3.0 ± 10) × 10^?11 at 25°C.     

Determination of chlorite at very low levels by using differential pulse polarography

A method is reported for the determination of μgl^?1 levels of chlorite by using differential pulse polarography. The electrochemical reduction of chlorite was studied between pH 3.7 and 14 and in an ionic strength range of 0.05–3.0 M. The optimum conditions are pH 4.1–4.4 and an ionic strength of 0.45 M. The current under these conditions is diffusion-controlled and is a linear function of chlorite concentration ranging from 2.77×10^?7 to 2.80×10^?4 M (19 μgl^?1 to 19 mg l^?1). The imprecision is better than ±1.0% and ±3.4% at concentrations of 2.87×10^?5 M and 1.74×10^?6M, respectively, with a detection limit of 1×10^?7 M (7μgl^?1). An interference study and the application of this method for determining chlorite in drinking water are reported.     

AUTOCATALYTIC REDUCTION OF PYRIDINECOBALOXIME(III) BY MOLECULAR HYDROGEN

Abstract

In the presence of added cobaloxime(II), hydroxopyridinecobaloxime(III) is autocatalytically reduced by molecular hydrogen in methanol at 20°C. The sigma-shaped volumetric curves were evaluated by computer simulation of the system of differential equations corresponding to a 4-step mechanism. The key reduction step is presumably H-atom transfer from hydridocobaloxime(III) to cobaloxime(III). The lower limit of its rate constant is k₄=(5.0 ± 0.5) × 10⁴ M^?1 sec^?1 at 20°C. Hydridopyridinecobaloxime(III) is thermodynamically unstable, its estimated formation equilibrium constant being (3.9 ± 0.6) × 10^?4 M^?1. The possible role of cobaloxime(I) species is discussed.     

Anionic polymerization with alkaline earth counterions—I. Propagation kinetics of living poly-2-vinylpyridine

Propagation kinetics of the homopolymerization of 2-vinylpyridine initiated with cumyl barium and with difunctional dimeric 1,1-diphenylethyl barium in tetrahydrofuran (THF) are reported. Conductance studies on the monofunctional living polymer solution, at concentrations from 10^?3 to 10^?5 M over the range 15 to ?70°C, revealed the presence of triple ions in thermodynamic equilibrium with free ions and ion pairs. The dissociation constant of ion pairs decreases from 1 × 10^?11 M at 15°C to 0.7 × 10^?11 M at ?70°C with an enthalpy of dissociation of 2.5 kJ/mol. Comparison with the analogous system of monofunctional polystyryl barium led to the conclusion that the living polymer of 2-vinylpyridine is 10 times less dissociated. Occurrence of intramolecular complexation, as observed with sodium and magnesium cations, thus appears much less pronounced in the presence of the barium cation on account of its relatively large radius. Kinetic studies showed that living poly-2-vinylpyridine, irrespective of its functionality, propagates essentially via ion pairs. The corresponding constant of propagation ranges from 294 M^?1 sec^?1 at 22°C to 20.7 M^?1 sec^?1 at ?50°C with an activation energy of 20 kJ/mol. The polymers contain more than 50%, isotactic triads.     

Polymerization of methyl methacrylate by a charge transfer mechanism with urea and iron (III) complex

Methyl methacrylate (MMA) can be polymerized by a charge transfer complex formed by the interaction of urea, methyl methacrylate, and carbon tetrachloride (CCl₄) in a nonaqueous solvent like dimethylsulfoxide (DMSO). The rate of polymerization can be accelerated by Lewis acids like Fe³⁺. This article reports the polymerization of MMA initiated by urea and CCl₄ and accelerated with hexakisdimethylsulfoxide iron (III) perchlorate, [Fe(DMSO)₆](ClO₄)₃, and A at 60°C. Definite induction periods were observed for the polymerization reaction initiated by urea and CCl₄ alone, but the induction period completely vanished when the molar ratio of urea to A reached 6:1. The molecular weights of the polymers with 6:1 molar ratio of urea to A were higher than with urea alone. The rate constant for the polymerization of MMA in the presence of [Fe(urea)₆]³⁺ was 1.03 × 10^?5 1 mol^?1 s^?1 at 60°C. The transfer constant for CCl₄ for polymerization with urea alone is 2.43 × 10^?3 at 60°C.     

Kinetics and mechanism of the inner-sphere reduction of hexachloroplatinate(IV) by dithionite in acetate buffer

The kinetics of reduction of hexachloroplatinate(IV) by dithionite have been examined spectrophotometrically in sodium acetate?Cacetic acid buffer medium in the temperature range 288?C303?K. The reaction is first order in both platinum(IV) species and dithionite. H⁺ ion has an inhibiting effect on the rate in the pH range 3.68?C4.80. The pseudo-first order rate constant increased upon increasing both ionic strength and dielectric constant. The suggested mechanism involves an initial transition state between two like charged ions, which then decomposes to give SO₃ ^2? through the intermediate formation of free radicals. The presence of free radicals was confirmed by performing the reaction in the presence of acrylamide. PtCl₆ ^2? is finally reduced to PtCl₄ ^2?, as confirmed by thermogravimetric analysis and IR spectrophotometry. The values of ?H^?? and ?S^?? associated with the rate-determining step have been calculated as 33?±?4?kJ?mol^?1 and ?141?±?7?JK?mol^?1, respectively. The values of ?H° and ?S° for the dissociation of HS₂O₄ ^? are 16?±?4?kJ?mol^?1 and ?14?±?7?JK?mol^?1, respectively.     

A novel spectrophotometric method for determination of cyanide using cobalt (II) phthalocyanine tetracarboxylate as a chromogen

Cobalt (II) phthalocyanine tetracarboxylate [Co (II)Pc-COOH] has been prepared and used in aqueous solutions as a novel chromogenic reagent for the spectrophotometric determination of cyanide ion. The method is based on measuring the increase in the intensity of the monomer peak in the reagent absorbance at 682 nm due to the formation of a 1 : 2 [Co (II)Pc-COOH] : [CN] complex. The complex exhibits a molar absorptivity (ε) of 7.7 × 10⁴ L mol^?1 cm^?1 and a formation constant (K_f ) of 5.4 ± 0.01 × 10⁶ at 25 ± 0.1°C. Beer's law is obeyed over the concentration range 0.15–15 µg mL^?1 (5.8 × 10^?6–5.8 × 10^?4 M) of cyanide ion, the detection limit is 20 ng mL^?1 (7.7 × 10^?7 M) the relative standard deviation is ±0.7% (n = 6) and the method accuracy is 98.6 ± 0.9%. Interference by most common ions is negligible, except that by sulphite. The proposed method is used for determining cyanide concentration in gold, silver and chromium electroplating wastewater bath solutions after a prior distillation with 1 : 1 H₂SO₄ and collection of the volatile cyanide in 1 M NaOH solution containing lead carbonate as recommended by ASTM, USEPA, ISO and APAHE separation procedures. The results agree fairly well with potentiometric data obtained using the solid state cyanide ion selective electrode.     

Solvation of ligandopentaamminecobalt (III) complexes in dimethyl sulfoxide–water mixtures

The rate constants for the replacement of water from the inner-coordination shell of Co(NH₃)₅OH₂³⁺, I, by dimethyl sulfoxide (DMSO) as DMSO gradually replaced water in the solvation shell of I were found to approach, and finally equal, the water-exchange rate constant of I in aqueous media in accordance with expectation for a dissociative mechanism. Also the rate constants for the replacement of DMSO from the innercoordination shell of Co(NH₃)₅DMSO³⁺, II, by water as water replaced DMSO in the solvation shell of II were found to approach, and approximately equal, the DMSO-exchange rate constant for II in liquid DMSO in accordance with expectation for a dissociative mechanism. The DMSO-exchange rate constant for II in liquid DMSO was determined and found to be equal to (3.6 ± 0.8) × 10^?4 sec^?1 at 45°C. The dissociation quotient, [II] [NO₃^?]/[Co(NH₃)₅NO₃²⁺], was found to be equal to 0.28 ± 0.11 M at 45°C by NMR methods. The pseudo first-order rate constants for anation of II by NO₃^? and the solvation of Co(NH₃)₅NO₃ ²⁺ by DMSO were determined at various temperatures.     

Gleichgewichte der Protonenaddition an 1, 1-Bis-(4′, 4″-dimethylaminophenyl)-äthylen in Methanol und in Dimethylsulfoxid

The first and second proton addition equilibrium constants of 1,1-bis-(4′, 4″-dimethylaminophenyl)-ethylene ( 1 ) have been measured by the spectrophotometric method in methanol and in dimethylsulfoxide. Defined as acid dissociation constants of the mono- and diprotonated adduct they are: K₁ (CH₃OH) = 8.3 (± 0.9) · 10^?6M, K₂ (CH₃OH) = 1.22 (± 0.06) · 10^?4M, K₁ (DMSO) = 2.3 (± 0.9) · 10^?3M, K₂ (DMSO) ≥ 1M. The evaluation of the electronic and the NMR. spectra demonstrates that the equilibrium of the two monoprotonated tautomers 2 (methyl-carbenium ion) and 3 (ammonium ion) is, in methanol to about 96% on the side of the ammonium ion (tautomeric equilibrium constant K₂₃ = [3]/[2] ? 23). The tautomer 2 cannot be detected in dimethylsulfoxide. The possible causes of these solvent effects are discussed.     

Kinetic studies of a catalyzed metalloporphyrin formation

Kinetics of the incorporation of mercury(II) ion in tetra (p-trimethylammoniumphenyl)porphine have been investigated in aqueous solution at 30.0°C and 0.2 M (NaNO₃) ionic strength. The reaction was found to be first order each in mercury(II) and the porphyrin. The forward (formation) and the reverse (dissociation) rate constants were found to be 1.9 ± 0.2 × 10³ M^?1 s^?1 and 7 ± 2 × 10⁶ M^?1 s^?1, respectively. Kinetics of zinc(II) incorporation in tetra(p-trimethylammoniumphenyl)porphine catalyzed by mercury(II) were also investigated. This catalysis is explained in terms of steady-state formation of mono mercury(II) porphyrin followed by zinc(II) displacement of mercury(II) ion from the porphyrin. Such a mechanism also illustrates the importance of porphyrin core deformation to metal incorporation.     

Homogeneous oxidation of hydrogen selenide with ozone in an argon-oxygen medium

Homogeneous oxidation of hydrogen selenide with ozone present in deficiency was studied. One of the reaction products is elemental selenium Se₈, which can be formed by multistep radical-chain polycondensation of hydrogen selenide. One mole of ozone is consumed per mole of hydrogen selenide. The observed rate constant of the reaction of hydrogen selenide with ozone at 20 ± 1°C is no less than (1.9 ± 0.2) × 10⁵ l mol^?1 s^?1.     

Calorimetric studies on the thermal decomposition of tri n-butyl phosphate-nitric acid systems

Thermal decomposition of neat TBP, acid-solvates (TBP·1.1HNO₃, TBP·2.4HNO₃) (prepared by equilibrating neat TBP with 8 and 15.6?M nitric acid) with and without the presence of additives such as uranyl nitrate, sodium nitrate and sodium nitrite, mixtures of neat TBP and nitric acid of different acidities, 1.1?M TBP solutions in diluents such as n-dodecane (n-DD), n-octane and isooctane has been studied using an adiabatic calorimeter. Enthalpy change and the activation energy for the decomposition reaction derived from the calorimetric data wherever possible are reported in this article. Neat TBP was found to be stable up to 255?°C, whereas the acid-solvates TBP·1.1HNO₃ and TBP·2.4HNO₃ decomposed at 120 and 111?°C, respectively, with a decomposition enthalpy of ?495.8?±?10.9 and ?1115.5?±?8.2?kJ?mol^?1 of TBP. Activation energy and pre exponential factor derived from the calorimetric data for the decomposition of these acid-solvates were found be 108.8?±?3.7, 103.5?±?1.4?kJ?mol^?1 of TBP and 6.1?×?10¹⁰ and 5.6?×?10⁹?S^?1, respectively. The thermochemical parameters such as, the onset temperature, enthalpy of decomposition, activation energy and the pre-exponential factor were found to strongly depend on acid-solvate stoichiometry. Heat capacity (C _p ), of neat TBP and the acid-solvates (TBP·1.1HNO₃ and TBP·2.4HNO₃) were measured at constant pressure using heat flux type differential scanning calorimeter (DSC) in the temperature range 32?C67?°C. The values obtained at 32?°C for neat TBP, acid-solvates TBP·1.1HNO₃ and TBP·2.4HNO₃ are 1.8, 1.76 and 1.63?J?g^?1?K^?1, respectively. C _p of neat TBP, 1.82?J?g^?1?K^?1, was also measured at 27?°C using ??hot disk?? method and was found to agree well with the values obtained by DSC method.     

Kinetics and mechanism of oxidation of aurate(I) by peroxydisulphate in aqueous hydrochloric acid

The reaction between Au(I), generated by reaction of thallium(I) with Au(III), and peroxydisulphate was studied in 5 mol dm^?3 hydrochloric acid. The reaction proceeds with the formation of an ion‐pair between peroxydisulphate and chloride ion as the Michealis–Menten plot was linear with intercept. The ion‐pair thus formed oxidizes AuCl₂^? in a slow two‐electron transfer step without any formation of free radicals. The ion‐pair formation constant and the rate constant for the slow step were determined as 113 ± 20 dm^?3 mol^?1 and 5.0 ± 1.0 × 10^?2 dm³ mol^?1 s^?1, respectively. The reaction was retarded by hydrogen ion, and formation of unreactive protonated form of the reductant, HAuCl₂, causes the rate inhibition. From the hydrogen ion dependence of the reaction rate, the protonation constant was calculated to be as 0.6 ± 0.1 dm³ mol^?1. The activation parameters were determined and the values support the proposed mechanism. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 589–594, 2002     

Kinetics of the mercury(II)‐catalyzed substitution of coordinated cyanide ion in hexacyanoruthenate(II) by nitroso‐R‐salt

The kinetics and mechanism of Hg²⁺‐catalyzed substitution of cyanide ion in an octahedral hexacyanoruthenate(II) complex by nitroso‐R‐salt have been studied spectrophotometrically at 525 nm (λ_max of the purple‐red–colored complex). The reaction conditions were: temperature = 45.0 ± 0.1°C, pH = 7.00 ± 0.02, and ionic strength (I) = 0.1 M (KCl). The reaction exhibited a first‐order dependence on [nitroso‐R‐salt] and a variable order dependence on [Ru(CN)₆^4?]. The initial rates were obtained from slopes of absorbance versus time plots. The rate of reaction was found to initially increase linearly with [nitroso‐R‐salt], and finally decrease at [nitroso‐R‐salt] = 3.50 × 10^?4 M. The effects of variation of pH, ionic strength, concentration of catalyst, and temperature on the reaction rate were also studied and explained in detail. The values of k₂ and activation parameters for catalyzed reaction were found to be 7.68 × 10^?4 s^?1 and E_a = 49.56 ± 0.091 kJ mol^?1, ΔH^≠ = 46.91 ± 0.036 kJ mol^?1, ΔS^≠ = ?234.13 ± 1.12 J K^?1 mol^?1, respectively. These activation parameters along with other experimental observations supported the solvent assisted interchange dissociative (I_d) mechanism for the reaction. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 215–226, 2009     

Studies of Thermal Transesterification of Phenylurethane by High Pressure Liquid Chromatograph

Thermal transesterification of phenylurethane with n-octanol was carried out in DMSO. It was found that the reaction followed first-order kinetics with an average rate constant of 8.630 × 10^?5 sec ^?1at 140°C. High pressure liquid chromatograph technique was employed to analyze chemical species in the course of the reaction. The reaction obeyed Arrhenius equation closely between 133°C and 155°C with activation energy of 29.6 kcal/mole and entropy of activation of ?8.2 cal/mole deg at 140°C. An intramolecular cyclic intermediate mechanism was proposed for this reaction.     

Charge transfer polymerization of 2-vinyl pyridine initiated by n-butyl amine and carbon tetrachloride

2-Vinyl pyridine (2-VP) can be initiated by a charge-transfer complex formed by the interaction of aliphatic amines such as n-butylamine (nBA) and carbon tetrachloride (CCl₄) in a solvent like NN-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). This article describes the polymerization of 2-VP by n-butylamine (nBA) in the presence of carbon tetrachloride in DMSO at 60°C. The rate of polymerization R_p increases rapidly with carbon tetrachloride (CCl₄) up to a concentration of 3.93 mol/L, but for a higher concentration it is almost independent of the carbon tetrachloride concentration; R_p is proportional to [nBA]^0.5 and [2-VP]^1.5 when [CCl₄]>[nBA]. The average rate constant k is 1.03 × 10^?5 L/mol s. When [CCl₄] < [nBA] the rate constant in terms of [2-VP] was 1.06 × 10^?5 s^?1 at 60°C and the overall rate constant was 1.035 × 10^?5 L/mol s at 60°C.     

Stopped-flow determination of carbon dioxide–diethanolamine reaction mechanism: Kinetics of carbamate formation

The reaction mechanism of carbon dioxide with diethanolamine (DEA) is investigated using the stopped-flow method with optical detection in the ranges of concentration [DEA] = 0.111–8.4 × 10^?2M and [CO₂] = 2.94–5.6 × 10^?3M. The comparison of the fast time-dependent light transmission change of a pH indicator with theoretical simulations of integrated rate equations requires a kinetic model in which a simple carbamate formation takes place simultaneously with hydration reactions, whose contributions are far from being negligible. A first-order reaction relative to DEA is thus found with a rate constant for carbamate formation smaller than usually predicted (110 ± 15M^?1s^?1 at 25°C). The equilibrium constant for the same reaction is also determined giving pK_R = 5.3 at 25°C, in satisfactory agreement with values assumed so far.     

The application of 2,4,6-trinitrobenzene-1-sulphonic acid in the differential pulse polarographic determination of amines

The differential pulse polarographic behaviour of 2,4,6-trinitrophenyl (TNP) derivatives of several primary amines and amino acids was investigated in the presence of sulphite ion. All the derivatives produced a polarographic peak for their complexes with sulphite (1 × 10^?2 M) in pH 8.0 phosphate buffer (0.05 M)/0.1 M potassium chloride. The derivatives of proteins and peptides did not give such a peak. A 5-min reaction time at room temperature (or 50°C for lysine) and pH 10.5 using 1 × 10^?4 M 2,4,6-trinitrobenzene-1-sulphonic acid provides the optimal conditions for the determination of 5 × 10^?6?2.5 × 10^?5 M amines. The relative standard deviation for determining 1 × 10^?5 M glycine (n = 5) was 1%.